A child hears it said that the stove is hot. But it is not, he says; and, indeed, that central body is not touching it, and only what that touches is hot or cold. But he touches it, and finds the testimony confirmed in a striking way. Thus, he becomes aware of ignorance, and it is necessary to suppose a self in which this ignorance can inhere. …

In short, error appears, and it can be explained only by supposing a self which is fallible.

Ignorance and error are all that distinguish our private selves from the absolute ego of pure apperception.